I'll just come right out and say it. This is my favorite book. I think the first time I read it I was in fourth grade and my dad had an old copy of the book wedged in the back of his classroom bookshelf. I didn't understand it all at the time, but I remember loving Tock, Faintly Macabre, and the Dodecahedron. Now, many years later it is still my favorite book. I've read it quite a few times since then, and I feel like I understand it much better than I did in the past, though with every subsequent reading I feel like I appreciate something new about the book. It's such an extensive adventure that there's always some new facet to explore.
I think my favorite part has to be when Milo steps in for Chroma the Great. I am enthralled every time I read about the Colorful Symphony. The idea is so captivating. Of course, as things begin to go wrong for him I feel the anxiety and worry that Milo feels as well, and I am oh, so grateful when Chroma returns. All in all, that particular scene's power comes from the vivid description of a creation so utterly impossible and simultaneously believable. I really feel that I live as Milo when I read that. Regarding the book as a whole, I'd call it one of the greatest pieces of children's fantasy fiction ever written, and certainly the best by an American author. It's one of those books that you put down at the end saddened, not because of what you've read, but because you've only been given so brief a glimpse into that other world that you've loved so well.
Norton Juster has written other books since then, some that have earned him great recognition and acclaim, but for me the classic of his career will always be The Phantom Tollbooth. You just don't get two chances at greatness like that in one lifetime.